The reliability of wireless (radio) communications depends heavily on characteristics of underlying wireless channels. When the channel characteristics are good, reliable communications can be achieved, but when the channel characteristic are bad, or more specifically when fade occurs in the channels, and the signal to noise ratio (SNR) is low, communications fails. Depending on a location and mobility of the transmitter and the receiver, and the environment of the channel, the channel characteristic can vary slowly or quickly.
To improve reliability, a number of diversity techniques are known. When sufficient frequency bandwidth is allocated to a network, a frequency diversity technique can be used by repeating a transmission in different frequency bands that have relatively low correlation with respect to each other. However, frequency diversity is impossible in certain channel environment with high coherent bandwidth compared to an allowable transmission bandwidth.
If the channel environment varies quickly, time diversity technique can be used by repeating a transmission at different times because the channel realizations have relatively low correlation with each other. However, time diversity is impossible in certain slowly varying channels compared to the latency constraint of transmission. That is, if the wireless channels at disjoint time intervals are strongly correlated, a failed transmission at a time interval implies failed transmission also at another interval nearby. In other words, time diversity cannot be achieved.
When the transmitter and the receiver have multiple transmit or receive antennas, space diversity can be achieved by transmitting data via different antennas, and combining the data appropriately at the receiver. Reliability can be improved as long as the transmit antennas and the receive antennas are spatially separated.
To reduce the cost of multiple antenna networks, an antenna selection diversity technique can be used at the transmitter and the receiver. Specifically, conventional antenna selection estimates channels characteristics between various transmit and receive antennas, and uses a subset of the available antennas that have optimal channel characteristics. Antenna selection diversity based on channel characteristics requires feedback to select the subset of optimal antennas, which increases overhead, latency, and power consumption.
Other ways of improving reliability of transmission can be achieved in a medium access control layer (MAC), such as automated repeat request (ARQ) and hybrid ARQ (HARQ). Those techniques require feedback from the receiver to transmitter to indicate whether a transmission is received successfully, and a retransmission is performed when a previous transmission fails. However, the use of feedback increases latency of the transmission.
It is desirable to implement a wireless network at a low cost and power consumption, with limited bandwidth, while still achieving high reliability and low latency.